1. Field of the Invention
The present invention relates to X-ray diagnostic installations, and in particular to such installations employing an X-ray tube for emitting a fan-shaped radiation beam, a means for sweeping the radiation beam across a subject which is disposed transversely to the direction of the radiation beam, an X-ray image intensifier and an image recording device which includes a display unit.
2. Description of the Prior Art
Installations of the type described above are employed for the production of synoptic radiographs. This radiography technique is generally referred to as Scanned Projection Radiography or Computer Radiography (CR).
An X-ray diagnostic installation of this type is described in German patent application P 30 23 401.6. That installation includes a primary radiation diaphragm or collimator mounted on an X-ray tube. A fan-shaped radiation beam is deflected so as to move across a patient and sweeps the input fluorescent screen of an X-ray image intensifier. A linear radiation image is produced on the input fluorescent screen which is transmitted to the output fluorescent screen of the intensifier. A television camera connected to the image intensifier picks up the successive linear images which are converted in a measurand converter into an X-ray shadow image which is displayed on a monitor. In order that only the useful signal, and not a signal produced by stray X-rays, is scanned, a cancelling device is utilized which effects a cancellation of stray X-rays on the target of the television camera directly before the line image. For this purpose the television camera includes a second electron gun for a second scanning beam which proceeds in advance of the image-forming scanning beam. The same result may be achieved with the use of only a single scanning beam, however, a portion of the scanning time must be dedicated to the cancellation of the regions bounding the line image.